With the spread of the Internet and a mobile communication, a carrier network provided by a common carrier is being transferred from a TDM network such as SONET/SDH (synchronous optical network/synchronous digital hierarchy) etc. to a packet network using an Ethernet (registered trademark) technique and an IP technique.
In the carrier network, a bandwidth guarantee has been put into practical use as one of the services provided for a user. In the bandwidth guarantee service, a maximum bandwidth (or a maximum transmission speed) determined based on, for example, a contract etc. is assigned to each user. In this case, a communication device which provides the bandwidth guarantee service performs bandwidth control so that the traffic of each user does not exceed a corresponding maximum bandwidth.
The bandwidth control is realized by, for example, data amount control. In this case, an amount of data per unit time is controlled. The amount of data may be expressed by bit/second (bps). The transmission device which relays a frame on a network (for example, a router) performs a routing process according to the address information included in the header of each frame. Therefore, when the number of frames per unit time increases, an overflow may occur in a circuit in the transmission device. Thus, the transmission device may control the number of frames. In the control of the number of frames, the number of transmission frames per unit time is limited not to exceed an allowed number.
The above-mentioned bandwidth control is realized using, for example, a token bucket. A token is provided at a specified rate for a token bucket. If the number of tokens accumulated in the token bucket exceeds a specified value (for example, zero), then a transmission control circuit reads a frame from a frame buffer and transmits the frame. In this case, the transmission control circuit reduces from the token bucket the number of tokens corresponding to the length of the frames read from the frame buffer. Therefore, the amount of data is controlled depending on the number of tokens supplied to the token bucket.
The related technique is described in, for example, WO2004/109995, Japanese Laid-open Patent Publication No. 2009-10449, Japanese Laid-open Patent Publication No. 2009-147569, and Japanese Laid-open Patent Publication No. 2011-259329.
In the bandwidth control using a token bucket, a burst transmission (the state in which the amount of data or the number of frames transitorily increases) may occur. Here, as described above, a token is supplied to the token bucket at a specified rate. Therefore, while a transmission device is not receiving a frame, tokens are accumulated in the token bucket. Afterwards, if the transmission device continuously receives frames (by the burst transmission), a transmission control circuit continuously outputs a frame until the number of tokens accumulated in the token bucket becomes is reduced to a specified value. Then, an overflow may occur in a frame processing circuit which processes the frames and/or a downstream node.
This problem may be solved by, for example, reducing the capacity of the token bucket (the number of tokens which may be accumulated in the token bucket). However, if the capacity of the token bucket is reduced too much, there is the possibility that the communication speed of a user does not reach the rate corresponding to a specified bandwidth.